In a hydroplant, the energy available is dependent on the water flow rate and the available pressure head. The head is the difference in height between the level of the body of water that feeds the turbine and the level of the turbine's outlet point. Normally, a dam is placed on a stream to increase the height of the body of water and also to build up the storage capacity of the body. In this way, the hydroplant can run practically continuously with stoppages only for drought or flooding conditions or for equipment maintenance/repair.
One normally sizes the turbine for continual operation, i.e. a small pond/stream will drive a small turbine. For very small streams, there has not been any practical method for harnessing the available energy since the flow rate of the stream cannot build up a pond that can provide continuous operation of commercially available hydroplants.